I have a card in my 830 that is capable of 90/80MB/s and in bechmark tools it will consistently give speeds around 50MB/s in the phone, in a usb3 reader it gives the full promised speed, ergo the phone caps out at around 50. The reason i recomend a card that can give at or around 50MB/s is that it will speed up many things, one is when you take photos directly to the card, with a slower card this increases time between shots. Anothers scenario might be when transfering data to and from the card through phone usb, this will cap out at ~32MB/s. Another would be if you play games from the card, a quick card will speed up loadtimes alot. An important thing to remember is that storage speeds vary alot depending on the size of data and how it's handled by the controller.
Another reason to pick a fast card is that the cost is usually the same as a slow one...